1. Field of the Invention
The present invention relates to a premix burner for operation in a combustion chamber, preferably in combustion chambers of gas turbines.
An exemplary field of application for a burner of this type is in the gas and steam turbine construction.
2. Brief Description of the Related Art
From patent document EP 0 321 809 B1, a conical burner is known that consists of a plurality of shells, a so-called double-cone burner. The conical swirler, which is composed of a plurality of shells, creates a closed swirling flow in a swirl chamber which, due to the swirl increasing in the direction of the combustion chamber, becomes unstable and transitions into an annular swirling flow with a backflow in the center. The shells of the swirler are assembled such that tangential air inlet slots for combustion air are formed along the burner axis. Along the inlet flow edge of the conical shells at these air inlet openings, feed lines for the premix gas, i.e., the gaseous fuel, are provided, which incorporate injection openings for the premix gas that are distributed along the direction of the burner axis. The gas is injected through the injection openings or bores crosswise to the air inlet slot. This injection process, in combination with the swirl of the combustion-air-fuel-gas generated in the swirl chamber, results in a good mixing of the premix fuel with the combustion air. In premix burners of this type a good mixing is the precondition for low NOx values during the combustion process.
To further improve a burner of this type, a burner for a heat generator is known from patent document EP 0 780 629 A2, which incorporates an additional mixing path following the swirler, for an additional mixing of the fuel and combustion air. This mixing path may be implemented, for example, in the form of a downstream tube section, into which the flow emerging from the swirler is transferred without any significant flow losses. With the aid of the additional mixing path the degree of mixing can be increased further and the pollutant emissions reduced accordingly.
Patent document WO 93/17279 shows an additional known premix burner, wherein a cylindrical swirler with a conical inner body is used. In this burner the premix gas is also injected into the swirl chamber via supply lines with corresponding injection openings that are arranged along the air inlet slots that extend in an axial direction. The burner additionally incorporates in its conical inner body a central feed line for burnable gas, which can be injected near the burner port into the swirl chamber for piloting. The additional pilot stage serves for the start-up of the burner, as well as to expand the operating range. In the so-called pilot operation, which incidentally also belongs to the generally known prior art for other premix-type burners, the fuel is injected in such a way—for example in the form of a gas jet that is injected along the burner axis—that it does not mix with the combustion air prior to the combustion process. This generates a diffusion flame which, even though it does result in higher pollutant emissions on the one hand, also has a significantly wider stable operating range on the other hand.
From patent document EP 1 070 915 A1, a premix burner is known wherein the burnable gas supply is mechanically decoupled from the swirler. This prevents tensions from thermal expansions when fuel gases are used that are not prewarmed or only slightly prewarmed. The swirler in this case is provided with a series of openings through which the fuel lines for the gas premix operation, which are mechanically decoupled from the swirler, extend into the interior of the swirler where they supply gaseous fuel to the swirling flow of the combustion air.
These known premix burners of the prior art are so-called swirl-stabilized premix burners, wherein a flow of a fuel mass is distributed as homogeneously as possible in a combustion-air mass flow prior to the combustion. The combustion air in these burner types flows into the swirlers via tangential air inlet slots. The fuel, particularly natural gas, is typically injected along the air inlet slots.
In gas turbines, synthetically produced gases, so-called Mbtu and Lbtu gases, are also used for combustion besides natural gas and liquid fuel, usually diesel oil or Oil#2. These synthesis gases are produced by gasifying coal or oil residues. They are characterized in that they largely consist of H2 and CO. Added to this is a smaller percentage of inert gases, such as N2 or CO2.
When synthesis gas is used for the combustion, the injection process that has proven effective for natural gas in the burners of the prior art can no longer be used because of a high danger of flashbacks.
The following particularities and requirements exist, in contrast to the use of natural gas, for a burner that is to be operated with synthesis gas. Synthesis gas requires a fuel volume flow that is approximately four times higher in dependence upon a dilution of the synthesis gas, which is known per se from the prior art—and in the case of undiluted synthesis gas even seven times higher or more—compared with comparable natural gas burners, so that noticeably different impulse conditions result with the same gas supply perforations of the burner. Due to the high content of hydrogen in the synthesis gas and the related low ignition temperature and high flame speed of the hydrogen, the fuel has a high propensity to react so that especially the flashback behavior and retention time of ignitable fuel-air mixture in the vicinity of the burner must be examined. Additionally, a stable and safe combustion of synthesis gases must be ensured for a sufficiently large range of heating values, which is composed differently depending on the process quality of the gasification and on the starting product, e.g., oil residues in the synthesis gas. In order to still be able under these conditions to attain a premixing and, along with it, the typical low emissions during the combustion process, these synthesis gases are usually diluted with inert gases, such as N2 or water vapor prior to their combustion. This reduces particularly the flashback danger that is immanent due to the high H2 content. The burner must thus be able to burn, in a safe and stable manner, synthesis gases of different compositions, especially different degrees of dilution, and the resulting significantly variable fuel volume flows.
Additionally it is advantageous if the burner can also safely burn a backup fuel in addition to the synthetic fuel. In the highly complex integrated gasification combined cycle (IGCC) systems, this requirement results from the demand for a high degree of availability. The burner should function safely and reliably in such a case also in a mixed operation of synthesis gas and backup fuel, for example diesel fuel, for which process the fuel mix spectrum for a single burner that can be used for the burner operation in a mixed operation must be maximized. Low emissions, typically NOx≦25 vppm and CO≦5 vppm, should, of course, be ensured for the specified and utilized types of fuel.
From patent document EP 0610 722 A1, a double-cone burner is known wherein a group of fuel injection openings for a synthesis gas are arranged on the swirler, distributed about the circumference of the burner at an end of the burner facing the combustion chamber. These injection openings are supplied via a separate fuel line and make it possible for the burner to be operated with undiluted synthesis gas.
However, this fuel injection at the combustion-chamber end of the burner can result in an insufficient mixing of the fuel with the swirling flow of the combustion air since the retention time of the fuel in the swirling flow prior to reaching the flame stabilizing zone (vortex recirculation zone) is short.
An additional problem arises with the above burners of the prior art if they are designed for the injection of a fuel with low to medium heating value, or if they are operated with such a fuel. Fuels with low to medium heating value must be injected into the swirling flow at high volume flows in order to achieve sufficient heat generation during the combustion. However, the high volume flows of the fuel disturb the swirling flow forming in the burner so that, in extreme cases, this can result in the flame-stabilizing recirculation zone failing to materialize.